A so-called “slug flow” capillary microreactor has been proposed for the investigation of mass transfer limited liquid-liquid reactions. Internal circulation within the slug leads to an intensified and tunable mass transfer. Understanding the development of the circulatory flows and the influence of operating parameters upon them is thus crucial. In this study, the experiments were carried out to visualise the internal circulations using Particle Image Velocimetry (PIV) technique. Also it uses the state-of-the-art computational fluid dynamics (CFD) simulations to predict the internal circulation within the liquid slugs and a CFD particle tracing algorithm to visualise them. Each slug was modelled as a distinct single-phase flow domain. The effect of flow velocity and slug length on the velocity profile and stagnant zones of the internal circulations for a slug with and without a wall film is discussed. The internal circulations could be qualitatively and quantitatively characterised with the help of the PIV measurements and particle tracing algorithm.